The present invention relates to a playback apparatus and playback method for reading marks or spaces written to an optical disc.
Writable optical discs are written with combinations of marks and spaces between marks. Mark length is 3T, 4T, 5T, 6T, 7T, 8T, 9T, 10T, or 11T, and space length is also 3T, 4T, 5T, 6T, 7T, 8T, 9T, 10T, or 11T. T denotes the period of the channel clock. The physical length of the shortest mark, 3T, is approximately 0.4 xcexcm, and the diameter (half width) of the light spot of the laser beam forming the marks is approximately 0.6 xcexcm. Because of the distribution of the light intensity of the light spot, that is, because light spot intensity is greatest in the center and weakens to the periphery, momentary exposure to the light spot forms slightly smaller 0.4 xcexcm marks instead of 0.6 xcexcm marks.
When marks are recorded to an optical disc two short marks could be recorded with a short space therebetween, such as when a 3T mark, 3T space, and 3T mark (denoted as 3Tm-3Ts-3Tm) are formed. When reproducing a 3Tm-3Ts-3Tm sequence thus recorded jitter occurs in the playback signal due to intersymbol interference, and 0 and 1 read errors occur.
FIG. 6 shows an optical disc playback apparatus according to the prior art. Reference numeral 62 is an optical disc, 64 is an optical head, 66 is an amplifier, 68 is an automatic gain controller, 70 is a capacitance, and 72 is a resistance; 74 is waveform equalizer for amplifying the frequency band of the reproduced signal, 76 is a digitizer, and 78 is a phase-locked loop.
As shown in FIG. 6, a waveform equalizer 74 is conventionally inserted before the digitizer 76 in order to resolve the above-noted intersymbol interference. The playback wave is amplified by the waveform equalizer 74. This operation is shown in FIG. 7.
FIGS. 7(a) and (b) show a signal reproduced from short marks and the reproduced signal after amplification by the waveform equalizer 74. FIGS. 7(c) and (d) show a signal reproduced from long marks and the reproduced signal after amplification by the waveform equalizer 74. The line shown in the middle of each signal denotes the threshold value Vth of the digitizer 76. A 1 is output from the digitizer 76 when this threshold value Vth is exceeded, and a 0 is output when less than or equal to the threshold value Vth. Jitter in the playback signal is thus reduced by the waveform equalizer 74 amplifying the high frequency band of the playback signal.
When a waveform equalizer 74 is not used, or when equalization coefficient K is weak, the point where the long mark playback signal intersects the threshold potential and the point where the short mark playback signal intersects the threshold potential can be matched as shown in FIG. 7(e) by inputting the optical disc playback signal directly to the digitizer 76, but if the optical disc playback signal is input to the digitizer 76 through a waveform equalizer 74 with a high equalization coefficient K, the point where the long mark playback signal intersects the threshold potential will be shifted offset g as shown in FIG. 7(f). This offset is a new source of jitter in the playback signal. This new jitter becomes increasingly pronounced as the recording density increases or the equalization coefficient K increases.
(Technical Problem to be Solved by the Invention)
An object of the present invention is to provide an optical disc playback apparatus capable of signal processing so that the above-noted new jitter does not appear even when a waveform equalizer is used.
(Method of Resolving the Problem)
A playback apparatus according to a first aspect of the present invention for reading marks or spaces (xe2x80x9cmarksxe2x80x9d below) written to an optical disc is characterized by comprising:
an optical head means for emitting a light spot to a mark, reading the mark, and outputting a playback signal;
a trimmer means for limiting at least the greater of positive amplitude or negative amplitude in the playback signal output from the optical head relative to a specified threshold value for converting to a binary signal so that both positive and negative amplitudes are substantially equal;
a waveform equalization circuit for amplifying the high frequency band of the output signal from the trimmer means; and
a digitizing circuit for converting the output signal from the waveform equalization circuit to a binary signal using the specified threshold value.
A second aspect of the invention is a playback apparatus according to the first aspect wherein the trimmer means limits amplitude in the positive direction only.
A third aspect of the invention is a playback apparatus according to the first aspect wherein the trimmer means limits amplitude in the positive direction and amplitude in the negative direction.
A fourth aspect of the invention is a playback apparatus according to the third aspect wherein the amount cut by limiting amplitude in the positive direction is 55% or less of the maximum positive amplitude, and the amount cut by limiting amplitude in the negative direction is 50% or less of the maximum amplitude in the negative direction.
A fifth aspect of the invention is a playback apparatus according to the third aspect wherein the trimmer means limits amplitude in the positive direction and amplitude in the negative direction in signals of an (n+2)T or longer mark length, and does not limit amplitude in the positive direction and amplitude in the negative direction in signals of an nT or (n+1)T mark length in modulation code where the shortest mark is nT (where n is a positive integer and T is the period of the channel clock).
A sixth aspect of the invention is a playback apparatus according to the first aspect wherein the trimmer means limits amplitude when the ratio ML/Wh between pit length ML on the optical disc and half width Wh of the light spot satisfies the following relationship:
ML/Whxe2x89xa7R
1.0xe2x89xa6Rxe2x89xa61.35.
A seventh aspect of the invention is a playback apparatus according to the first aspect wherein the ratio between the shortest mark length MLmin of the optical disc and the half width Wh of the light spot satisfies the relationship MLmin/Wh  less than 0.82.
An eighth aspect of the invention is a playback method for reading marks written to a writable optical disc, comprising:
a step for emitting a light spot from an optical head to a mark, reading the mark, and outputting a playback signal;
a step for limiting at least the greater of positive amplitude or negative amplitude in the playback signal output from the optical head relative to a specified threshold value for converting to a binary signal so that both positive and negative amplitudes are substantially equal;
a step for amplifying the signal of substantially equal positive and negative amplitudes; and
a step for converting the amplified signal to a binary signal using the specified threshold value.
A ninth aspect of the invention is a playback method according to the eighth aspect wherein the step for substantially equalizing positive and negative amplitudes limits amplitude in the positive direction only.
A tenth aspect of the invention is a playback method according to the eighth aspect wherein the step for substantially equalizing positive and negative amplitudes limits amplitude in the positive direction and amplitude in the negative direction.
An eleventh aspect of the invention is an optical disc playback method according to the tenth aspect wherein the amount cut by limiting amplitude in the positive direction is 55% or less of the maximum positive amplitude, and the amount cut by limiting amplitude in the negative direction is 50% or less of the maximum amplitude in the negative direction.
A twelfth aspect of the invention is an optical disc playback method according to the tenth aspect wherein the trimmer means limits amplitude in the positive direction and amplitude in the negative direction in signals of (n+2)T or longer mark length, and does not limit amplitude in the positive direction and amplitude in the negative direction in signals of nT or (n+1)T mark length when the shortest mark of the modulation code is nT (where n is a positive integer and T is the period of the channel clock).
A thirteenth aspect of the invention is an optical disc playback method according to the eighth aspect wherein amplitude is limited by the step for substantially equalizing positive and negative amplitudes when the ratio ML/Wh between pit length ML on the optical disc and half width Wh of the light spot satisfies the following relationship:
ML/Vhxe2x89xa7R
1.0xe2x89xa6Rxe2x89xa61.35.
A fourteenth aspect of the invention is an optical disc playback apparatus according to the eighth aspect wherein the ratio between the shortest mark length MLmin of the optical disc and the half width Wh of the light spot satisfies the relationship
MLmin/Wh less than 0.82.
(Benefits More Effective Than the Prior Art)
An optical disc playback apparatus and method capable of signal processing without producing jitter can be provided by means of the present invention.